Surface-mount semiconductor package

ABSTRACT

A surface-mount semiconductor package comprises a semiconductor which is embedded within a plastic material body. Electrical control connections to the semiconductor package include first and second pads which are spaced apart from each other, making it easier to couple the devices in parallel. Power output is provided by metal pads. The pads may be partially sheared, to step them, thereby allowing a single thickness lead frame to be used in the manufacture of the device. On the lower face of the body there are channels which increase the electrical tracking distance and allow improved washing of residues after the device has been secured to a substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface-mount semiconductor packageand, in particular although not exclusively to a power controller whichis arranged in use to be surface-mounted to a PCB (printed circuitboard) or an IMS (insulated metal substrate) board.

2. Related Art

A typical surface-mount semiconductor package comprises a semiconductordevice which is embedded within a molded plastic material body. A metalleadframe, also partially embedded within the body, electricallyconnects the semiconductor device with external legs which are solderedto a track on a PCB or an IMS board.

A typical prior art example is shown in section in FIG. 1. As will beseen, the device comprises a plastic material body 2 having a verticalfront face 4 from the center of which the legs 6 (of which one is shownin the drawing) emerge. The legs have a flat upper portion 8,downwardly-cranked portion 10, and a lower flat distal portion 12. Thedistal portion 12 is secured to the substrate 14, typically a PCB or anIMS board, by soldering (not shown).

A problem with this type of prior art device is that it can be difficultto manufacture the legs 6 to ensure that the distal portion 12 is flatand makes good contact with the substrate. If the legs become slightlybent, for example, or if they are not manufactured with reasonableaccuracy, electrical contact can be unreliable.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least alleviate thedifficulties of the prior art.

It is a further object to provide a relatively inexpensive surface-mountsemiconductor package which is easy to mount to an appropriatesubstrate.

According to a first aspect of the invention there is provided asurface-mount semiconductor package comprising a semiconductor devicemolded within a plastic material body, a pair of input electrodes forcontrolling the device and an output electrode providing an outputsignal from the device, the input and output electrodes extending out ofthe body; characterised in that the input electrodes are offset from oneanother.

According to a second aspect of the invention there is provided asurface-mount semiconductor package comprising a semiconductor devicemolded within a plastic material body (10), a pair of input electrodesfor controlling the device and an output electrode providing an outputsignal from the device, the input and output electrodes extending out ofthe body; characterised in that the body includes a channel on a lowersurface thereof, the set lower surface in use being arranged to contacta mounting substrate.

According to a third aspect of the present invention there is provided asurface-mount semiconductor package comprising a semiconductor devicemolded within a plastic material body (10), a pair of input electrodesfor controlling the device and an output electrode providing an outputsignal from the device, the input and output electrodes extending out ofthe body; characterised in that the output electrode comprises a padwhich, in use, is arranged to contact a conductive portion of a mountingsubstrate, the pad being partially sheared.

The input electrodes preferably comprise elongate legs which are bentout of a general plane of the package. The legs, which are offset fromone another, may initially form part of a planar blank (preferably of aconstant thickness), the legs being bent into position duringmanufacture. Alternatively, if the legs are not required, they maysimply be cut off.

It is preferred that in the blank the legs are generally parallel, andoverlap one another. Preferably, the points at which the legs merge intothe rest of the blank are adjacent to the outside edges of the blank. Byproviding overlapping legs, in this way, which preferably runtransversely across the blank, material is saved since there is no needto make the blank excessively long purely to provide enough materialfrom which the legs can be constructed.

Preferably, the legs form input (signal) electrodes to the semiconductordevice or, alternatively, the input electrodes could comprise pads. Oneor more output (power) electrodes may be provided, which may preferablybe in the form of pads which are arranged in use to be secured to anelectrically-conducting portion of a substrate, for example a PCB or anIMS board. Alternatively, the output electrodes may take the form oflegs.

The body of the semiconductor package is preferably molded of a plasticmaterial, thereby encapsulating the semiconductor device. On a lowersurface of the body may be formed a channel. In use, this forms apassage between the lower surface of the body and the mounting substratethrough which washing solutions may be directed. It is furtherpreferable for the channel to be positioned so that it forms part of anelectrical tracking path along the lower surface of the body. Forexample, the channel may be positioned between the output electrode orelectrodes, and a heat sink.

In the preferred embodiment, either the input electrodes or the outputelectrodes, or both, comprise pads which are, in use, arranged tocontact a conductive portion of a mounting substrate. The pads arepreferably partially sheared. This enables a single-thickness blank tobe used, either for the input electrodes or for the output electrodes,or both. On shearing, one part of the lower surface of each pad ispushed downwardly, forming a downwardly-directed contact surfacesuitable for electrical connection to the substrate.

Within the plastic material body is at least one (and perhaps severallinked) semiconductor devices. For example, the package could containboth a diode and a transistor.

The invention also extends to a surface-mount semiconductor packagecomprising a semiconductor device molded within a plastic material body,a pair of input electrodes for controlling the device and an outputelectrode providing an output signal from the device, the input andoutput electrodes extending out of the body; characterised in that theinput electrodes are at one end of the body and the input electrode isat an opposite end. Such an arrangement reduces electrical noiseproblems (e.g., mains-borne noise).

The invention extends to an array of surface-mount semiconductorpackages, each as previously described. The array may be connected inparallel using the offset feature of the input electrodes to permit oneof the input electrodes on each package to be connected to a firstsignal track, and the other input electrode on each package to beconnected to a second, parallel, signal track. The first and secondsignal tracks may be on a separate control board which may desirably bepositioned over the array. Where the input electrodes comprise elongatelegs, the legs may extend upwardly from the substrate, and may besecured to the first and second tracks of the control board. In thepreferred arrangement, there may be apertures in the control boardthrough which the legs extend. The distal ends of the legs are thensoldered or otherwise electrically connected to the tracks.

The present invention also extends to a method of manufacturing asurface-mount semiconductor package. It further extends to a method ofmounting such a package to a substrate, including the step of washingthe package. Still further, the invention extends to the assembly of anarray of such packages.

The invention further extends to any one or more compatible feature asset out above, or as referred to in the specific description, whethertaken alone or in any compatible combination.

It is further envisaged in the present invention that there may be onlyone input (or signal) electrode in some embodiments, instead of a pair.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be carried into practice in a number of ways and twospecific embodiments will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic sectional view of a prior art surface-mountsemiconductor package;

FIG. 2 shows perspective views of a surface-mount semiconductor packageaccording to a first aspect of the present invention;

FIG. 3 shows the way in which the packages of FIG. 2 may be mounted inparallel;

FIG. 4 shows a top view of the embodiment of FIG. 2, prior to bending ofthe legs of the leadframe;

FIG. 5 shows the leadframe for the embodiment of FIG. 2;

FIG. 6 is a top view of a second embodiment of the present invention;

FIG. 7 shows the leadframe for the embodiment of FIG. 6; and

FIG. 8 illustrates how the sheared parts of FIG. 6 are constructed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows a first embodiment of a surface-mount semiconductor packageaccording to the present invention. The package comprises a moldedplastic material body 10 within which (not shown) is embedded asemiconductor device. In the particular example shown, the semiconductordevice is a power controller, control signals to which are supplied viaa pair of upstanding metal legs 12, 14. Power output is provided, at theother end of the device, via a pair of metal pads 16, 18. On theunderside of the device there is a metal heat sink 20 which is notconnected electrically to the legs 12, 14, to the pads 16, 18 or to theembedded semiconductor device.

Molded on the underside of the body 10, between the heat sink 20 and thelegs 12, 14 is a transverse groove 22, having a centralforwardly-directed opening 28. Likewise, there is a similar groove 24between the rear edge of the heat sink 20 and the pads 16, 18. Thegroove 24 has a centrally-positioned rearwardly-directed opening 26which opens into a space between the pads 16, 18.

In use, the package is surface-mounted to a substrate, for example to aPCB or to an IMS board, with the heat sink 20 abutting a heat sink ofthe board, and the pads 16, 18 being soldered to power-carrying trackson the board. Control signals are supplied to the legs 12, 14 either bysoldering wires directly onto the upstanding portions of the legs, oralternatively by soldering the flat undersides of the legs 30, 32 toappropriate signal tracks on the board.

FIG. 3 shows how a plurality of devices as shown in FIG. 2 may bemounted and used in parallel. In FIG. 3, two such devices are shownmounted to an IMS board. The IMS board 33 comprises a heat-conductingsubstrate 34 (for example of aluminium), carrying anelectrically-insulating surface 36. On the surface are parallel coppertracks 38, 40. The devices are mounted so that the pads 16, 18 aresecured (for example by soldering) to the track 38, and with the heatsink 20 being secured to the track 40.

Above the row of devices is a control board 42 having a pair of coppersignal tracks 44, 46. Apertures in the board allow the upstanding legs12, 14 to pass through the board, thereby coming into contact withrespective enlarged contact portions 48, 50 of the tracks 46, 44. Goodelectrical contact is ensured by soldering the enlarged areas and thelegs together (not shown).

It will be noted that the legs 12, 14 are staggered, therefore ensuringthat all of the legs 12 will be in one horizontal line, and all of thelegs 14 will be in another parallel horizontal line.

In use, control signals are supplied along the tracks 44, 46simultaneously to all of the electronic devices. The power output arisesin the track 38 on the IMS board 33. It has previously been indicatedthat the heat sinks 20 are not electrically connected to anything, andthese will normally be coupled to ground (for example, earth ground) viathe copper track 40. Alternatively, in a variation of the embodiment,the heat sink 20 could represent one of the power output terminals, withthe pads 16, 18 representing the other terminal. In another variation(which would require a different displacement of copper tracks on theIMS board), the pad 16 could represent a first power output terminal,while the pad 18 represents the other power output terminal.

Because of the large voltages that may be generated on the track 38, itis important to ensure that there can be no sparking or breakdownbetween the track 38 and the track 40. The groove 24 acts to improve theelectrical insulation between the two tracks, in that it increases theelectrical tracking distance along the lower surface of the plasticsmaterial body 10. If the channel 24 were not present, and the lowerportion of the body were flat, the tracking distance would be lowerwhich would mean that a breakdown would happen at a lower voltage,particularly if the surface happens to become slightly damp.

The packages are normally secured to the IMS board by firstscreen-printing a mixture of flux and solder (solder paste) flux ontothe board. Difficulties can arise with this procedure, however, in thatone can get left with a certain amount of residue on the board.Furthermore, the solder can squeeze out sideways from underneath thepads 16, 18. With prior art devices, which lack a channel 24,squeezed-out solder can easily move across a large part of the spacebetween the track 38 and the track 40. With the provision of the channel24, on the other hand, any stray solder will normally be taken up intothe channel instead.

Once the devices have been secured to the IMS board, solder and residuecan be removed by passing washing fluid along the channels 22, 24. Thewashing fluid also passes through the openings 26, 28 (FIG. 2), therebyensuring that all of the areas around the pads 16, 18 and the lowerportions 30, 32 of the legs are clear of unwanted material.

FIG. 4 shows a top view of the package of FIG. 2, before the legs 12, 14have been bent upwardly. As will be seen, at this stage in themanufacture of the final product, the legs 12, 14 lie in a single plane,and can be bent as required into the configuration that is designed bythe end user. In some cases, the end user may prefer to supply a signalthrough tracks on the IMS board, and to access the semiconductor deviceby the lower portions 30, 32 of the legs. In that case, the legs maysimply be cut off.

The product shown in FIG. 4 is in fact a slight variant on the device ofFIG. 2, in that it includes a cut-away portion 56 in the rear edge ofthe body, and a like cut-away portion 58 in the forward edge. Thisenables the washing solutions to enter more easily into the channels 22,24.

FIG. 5 shows yet an earlier stage of the manufacturing process, namelythe leadframe which will eventually form the legs 12, 14 and the pads16, 18. The leadframe comprises a flat metal stamping, of substantiallyconstant thickness throughout. As manufactured, the leadframe is anelongate structure, only part of which is shown in the figure. Theleadframe is then cut up into individual portions, one for each package,along the cutting lines 52, 54. An electronic device is then attached tothe leadframe, which is then embedded within the plastic material body10 to form the product shown in FIG. 4.

FIG. 6 shows an alternative embodiment in which the legs 12, 14 arereplaced by pads 122, 144 which are substantially identical with thepads 16, 18. With this embodiment, control signals would normally beprovided to the underside of the pads 122, 144 by appropriate coppertracks on the PCB or IMS board to which the device is mounted.

FIG. 7 shows a portion of the leadframe suitable for manufacturing sucha device. Again, the cutting lines are indicated at 52, 54.

In this embodiment, the pads 16, 18, 122, 144 are partially sheared, asmay best be seen in FIG. 8. The shearing is carried out at the same timethat the leadframe is punched out, and is achieved by using offsetpunches 60, 62 which operate in exactly the same way as do a pair ofscissors. The movement of the punches is, however, stopped before thepad is sheared through.

It will be appreciated that this stepped arrangement could also be usedfor the pads 16, 18 of the embodiment of FIG. 2.

The advantage of providing a stepped pad, in this way, is that the lowersurface of the pad (62 in FIG. 8) can be positioned downwardly, out ofthe general plane of the leadframe, without the necessity ofmanufacturing the leadframe from a multi-thickness blank.

Devices of the types described may desirably have a length of about 30mm, and a width of about 18 mm. They may deal with power transfers of upto 100 watts at 40 amps.

What is claimed is:
 1. A surface-mount semiconductor package comprising:a flat conductive heat sink mount, a semiconductor device coupled to theheat sink mount, a molded plastic material body for enclosing thesemiconductor, a pair of input electrodes for controlling the device,and an output electrode providing an output signal from the device, theinput and output electrodes being displaced from and co-planar with theheat sink mount; wherein the input electrodes and output electrodes areat opposite ends of said package.
 2. A package as claimed in claim 1,wherein the input electrodes comprise elongated legs which are bent outof a general plane of the package.
 3. A package as claimed in claim 2,wherein the elongate legs are directed inwardly towards one another,with the distal ends of the legs being substantially parallel.
 4. Apackage as claimed in claim 1, wherein the input electrodes and heatsink mount are punched out of a blank of uniform thickness.
 5. Asurface-mount semiconductor package comprising: a flat conductive heatsink mount, a semiconductor device coupled to the heat sink mount, amolded plastic material body for enclosing the semiconductor device, apair of input electrodes extending from an edge of the heat sink mountfor controlling the devices, and an output electrode extending from theheat sink mount for providing an output signal from the device, theinput and output electrodes extending out of the body; wherein the bodyincludes a channel on a lower surface thereof between the heat sink andat least one electrode, the lower surface being planar and adapted tocontact a mounting substrate in use.
 6. A package as claimed in claim 5,wherein the channel extends across the lower surface between the outputelectrode and a further electrode.
 7. A package as claimed in claim 6,wherein the further electrode comprises a second output electrode.
 8. Apackage as claimed in claim 6, wherein the channel is adapted to form apassage between the body of the package and the mounting substratethrough which washing solutions may be directed.
 9. A package as claimedin claim 8, including two spaced-apart output electrodes, the channelhaving an aperture in a side thereof, the aperture opening between thetwo output electrodes.
 10. A package as claimed in claim 5, wherein thechannel has an aperture in a side thereof, the aperture opening betweenthe input electrodes.
 11. A surface-mount semiconductor packagecomprising: a semiconductor device molded within a plastic materialbody, a pair of input electrodes for controlling the device and anoutput electrode providing an output signal from the device, the inputand output electrodes extending out of the body; wherein the outputelectrode comprises a partially sheared pad adapted to contact aconductive portion of a mounting substrate.
 12. A package as claimed inclaim 11, wherein the input electrodes also comprise pads which arepartially sheared.
 13. A package as claimed in claim 12, wherein eachpad is punched out of a blank.
 14. A package as claimed in claim 13,wherein the blank is of a substantially uniform thickness.
 15. Asurface-mount semiconductor package comprising: a semiconductor devicemolded within a plastic material body having a bottom plane, a pair ofinput electrodes having lower surfaces, the input electrodes forcontrolling the device, and an output electrode having a lower surface,the output electrode providing an output signal from the device, thelower surfaces of the input and output electrodes being substantiallyco-planar with the bottom plane, the electrodes extending out of thebody; wherein the input electrodes are at one end of the body and theoutput electrode is at an opposite end of the body.
 16. A package asclaimed in claim 15, wherein the body is generally rectangular, with theinput electrodes being adjacent a shorter side of the body and theoutput electrode being adjacent an opposing shorter side of the body.17. A package as claimed in claim 16, wherein the input electrodes aresignal electrodes and the output electrode is a power electrode.
 18. Apackage as claimed in claim 15, including a plurality of semiconductordevices.
 19. A surface-mount semiconductor package comprising a thinflat rectangular conductive heat sink mount having parallel upper andlower surfaces, a semiconductor device mounted on said upper surface ofsaid thin flat conductive heat sink; a pair of output electrodesdisposed adjacent a first side of said thin flat heat sink mount andspaced therefrom and coplanar therewith; an input electrode adjacent asecond side of said thin flat heat sink which is opposite and parallelto said first side and spaced therefrom and coplanar therewith; a moldedplastic housing enclosing said semiconductor device and said uppersurface of said thin flat conductive heat sink and said first and secondsides thereof and at least first portions of said first and secondoutput electrodes and said input electrode which are adjacent said firstand second sides; said output electrodes and said input electrode havingsecond portions which extend beyond the periphery of said molded plastichousing and being available for connection to external conductors; saidlower surface of said thin flat conductive heat sink being exposed forsurface-to-surface connection to an external flat heat sink.
 20. Thesurface-mount package of claim 19 wherein said output electrodes aredisposed at respective adjacent corners of said package.
 21. Thesurface-mount package of claim 20 wherein said output electrodes areconnected together and are connected to the top of said semiconductordevice.
 22. The surface-mount package of claim 19 which further includesa second input electrode displaced from said input electrode and spacedfrom and coplanar with said thin flat conductive heat sink.
 23. Thesurface-mount package of claim 20 which further includes a second inputelectrode displaced from said input electrode and spaced from andcoplanar with said thin flat conductive heat sink.
 24. The surface-mountpackage of claim 21 which further includes a second input electrodedisplaced from said input electrode and spaced from and coplanar withsaid thin flat conductive heat sink.
 25. The surface-mount package ofclaim 19 which contains first and second grooves extending from thebottom surface of said molded plastic housing and parallel to said firstand second sides respectively and disposed between and spaced from saidfirst and second sides and said first and second output electrodes andsaid input electrode respectively; said first and second groovesincreasing the tracking distance along the surface of said moldedhousing between said first and second sides and said first and secondelectrodes and said input electrode respectively; said first and secondgrooves further serving as washing channels during solder-down of saidpackage to a support substrate.
 26. The surface-mount package of claim25 wherein said output electrodes are disposed adjacent respectivecorners of said package.
 27. The device claim 19 wherein said first andsecond portions of said first and second output electrodes and saidinput electrode are partially vertically displaced from one another toimprove their locking into said molded plastic housing.
 28. The deviceof claim 27 wherein said vertical displacement is produced by thepartial shearing of said first and second portions.
 29. The device ofclaim 25 wherein said first and second portions of said first and secondoutput electrodes and said input electrode are partially verticallydisplaced from one another to improve their locking into said moldedplastic housing.
 30. The device of claim 29 wherein said verticaldisplacement is produced by the partial shearing of said first andsecond portions.
 31. The device of claim 19 wherein said package is anelongated rectangle, said first and second sides being shorter than thesides perpendicular thereto.
 32. The device of claim 20 wherein saidpackage is an elongated rectangle, said first and second sides beingshorter than the sides perpendicular thereto.
 33. The device of claim 25wherein said package is an elongated rectangle, said first and secondsides being shorter than the sides perpendicular thereto.
 34. The deviceof claim 28 wherein said package is an elongated rectangle, said firstand second sides being shorter than the sides perpendicular thereto.